1. Field of the Invention
The present invention relates to a rotating field sensor for detecting an angle that the direction of a rotating magnetic field forms with respect to a reference direction.
2. Description of the Related Art
In recent years, rotating field sensors have been widely used to detect the rotational position of an object in various applications such as detecting the rotational position of an automotive steering wheel. Rotating field sensors are used not only to detect the rotational position of an object but also to detect a linear displacement of an object. Systems using rotating field sensors are typically provided with means (for example, a magnet) for generating a rotating magnetic field whose direction rotates in conjunction with the rotation or linear movement of the object. The rotating field sensors use magnetic detection elements to detect the angle that the direction of the rotating magnetic field forms with respect to a reference direction. The rotational position or linear displacement of the object is thus detected.
There has been known a rotating field sensor that has two bridge circuits (Wheatstone bridge circuits) as shown in U.S. Pat. No. 6,633,462 B2, U.S. Pat. No. 5,363,034, U.S. Pat. No. 5,216,363, U.S. Pat. No. 6,465,053 B1, and U.S. Pat. No. 6,501,678 B1. In such a rotating field sensor, each of the two bridge circuits includes four magnetoresistive elements (hereinafter, also referred to as MR elements) serving as magnetic detection elements. Each of the bridge circuits detects the intensity of a component of the rotating magnetic field in one direction, and outputs a signal that indicates the intensity. The output signals of the two bridge circuits differ in phase by ¼ the period of the output signals of the bridge circuits. The angle that the direction of the rotating magnetic field forms with respect to a reference direction is calculated based on the output signals of the two bridge circuits.
In a rotating field sensor that uses an MR element as a magnetic detection element, the waveform of the output signal of the MR element corresponding to the resistance ideally traces a sinusoidal curve (including a sine waveform and a cosine waveform) as the direction of the rotating magnetic field rotates. However, it is known that the output signal waveform of an MR element can be distorted from a sinusoidal curve, as described in U.S. Pat. No. 6,633,462 B2. Distortion of the output signal waveform of an MR element from a sinusoidal curve means that the output signal of the MR element includes a harmonic component other than a fundamental sinusoidal component, as described in U.S. Pat. Nos. 5,363,034 and 5,216,363. The harmonic component included in the output signal of the MR element may cause an error in the angle detected by the rotating field sensor. The error is mainly caused by a second and a third harmonic component.
U.S. Pat. No. 6,633,462 B2 discloses a magnetoresistive sensor including a main sensing element having a main reference magnetization axis, and two correction sensing elements having their respective reference magnetization axes inclined with respect to the main reference magnetization axis. The two correction sensing elements are electrically connected to the main sensing element to correct the detected angle. The two correction sensing elements output signals that are out of phase with each other by a half period of an error signal of the main sensing element. In this sensor, the output signals of the two correction sensing elements are added to the output signal of the main sensing element. This allows for eliminating at least part of the error signal of the main sensing element. The sensor disclosed in U.S. Pat. No. 6,633,462 B2, however, has a drawback of increasing in size because the two correction sensing elements are required in addition to the main sensing element. Further, the sensor requires that the resistances of the correction sensing elements be set to an optimum value different from that of the main sensing element. This poses a problem that it is not easy to design and manufacture this sensor.
U.S. Pat. Nos. 5,363,034 and 5,216,363 disclose a technique for reducing harmonic components of the output signal of a magnetic sensor that is disposed to face a magnetic scale in which magnets are arrayed with a predetermined pitch. According to the technique, the harmonic components are reduced by serially connecting a plurality of magnetoresistive elements that are spaced apart from each other by a predetermined distance in the direction in which the magnets are arrayed. However, this technique requires that the arrangement of the plurality of magnetoresistive elements be changed according to the array pitch of the magnets because the arrangement of the magnetoresistive elements depends on the array pitch of the magnets. For this reason, this technique has the problem that it cannot be applied to the case where the magnets are arrayed with an arbitrary pitch.